Coatings for stator end windings of turbine generators must aid in absorbing vibration and in helping to secure the end windings, so that they will not move relative to one another during operation of the generator. In addition, the coating must have outstanding abrasion resistance, toughness, flexibility and thermal aging characteristics. It should also be nontoxic, easily applied in thick sections, curable at about 25.degree. C. within about 10 hours, have a low odor, and be attractive in order to provide a cosmetic covering over the end winding assembly.
Bracing coatings have been tested for this application containing about 95 parts of bisphenol A epoxy resin; 5 parts of a diluent such as a diglycidyl ether of an aliphatic diol; 2 parts of Fe.sub.2 O.sub.3 pigment; and 20 parts of amine curing agent. Such coatings have lacked toughness and good thermal aging characteristics: the flexibility of 1/8 inch thick samples lasting only from about 1 to 10 days at 100.degree. C. before cracking upon being bent around a 2 inch diameter mandrel, with a weight loss at the time of failure of from about 1.5 percent to 20 percent.
In order to impart resiliency and toughness to normally rigid epoxy castings, Lee and Neville, in the Handbook Of Epoxy Resins, 1967, Chapter 16-16, reports the use of acrylonitrile-butadine polymers. B. F. Goodrich Chemical Co., in a 1976 product brochure entitled "Hycar ATBN Elastomers" describes secondary amine terminated butadiene/acrylonitrile (ATBN) reactive liquid polymers, having a 2-butenylene:cyanoethylene chain segment ratio of about 5:1, as tougheners for epoxy resins when used at levels below 10 parts/100 parts epoxy, as flexibilizers for epoxy resins when used at levels above 20 parts/100 parts epoxy, and as curing agents for epoxy resins when used at levels above 150 parts/100 parts epoxy.
These ATBN materials may be used with solvents, such as xylene, methyl ethyl ketone or toluene. Such epoxy-ATBN compositions, however, while substantially upgrading flexibility of 1/8 inch thick samples to about 90 days at 100.degree. C. before cracking upon being bent around a 2 inch diameter mandrel, with weight losses ranging from about 1.8 percent to 4.1 percent at the time of failure, still do not exhibit all of the essential qualities required of stator end winding coatings.
Riew, in U.S. Pat. No. 4,055,541, taught room temperature curable compositions containing non-cycloaliphatic epoxy and from 1 to 1,000 parts/100 parts epoxy of an amine-terminated ATBN liquid polymer. Catalyst and curing agents are optional, and can comprise BF.sub.3 -amine complexes, imidazoles and triethylenetetramine. Up to 50% filler is allowed and solvents such as kerosine, toluene, benzene and the like can be used. Fillers include carbon blacks, carbonates, silicates, glass, asbestos, textile fibers, metal oxide colorants. Plasticizers, such as castor oil, silicones, phthalates, sebacates and trimellitates are also taught. Suggested uses include spraying, casting, painting or dipping applications, to provide 1 to 100 mil coatings on substrates such as tire sidewalls. Other uses include molded gears, appliance housings and electrical circuit boards.
What is needed, however, is a composition formulated specifically as a solventless stator adhesive-bracing composition. This composition must have outstanding toughness, flexibility, thermal aging, insulating and short cure characteristics. For example, continued flexibility of 1/8 inch thick samples to over about 250 days at 100.degree. C. before cracking upon being bent around a 2 inch diameter mandrel, weight losses of below about 1.25 percent after 250 days, and 10 hour curing properties at 25.degree. C. The material should also be sprayable in thick coatings.